On the use of slant observations from GPS to diagnose three dimensional water vapor using 3DVAR

.The use of ground based GPS receivers to diagnose precipitable water in the vertical column has been demonstrated. Recent results indicate that the integrated slant precipitable water between ground-based GPS receivers and GPS satellites in view can also be diagnosed. This raises the possibility that a network of GPS receivers could provide enough information through slant water vapor measurements to recover significant detail about the three dimensional moisture field. To test this concept, a simple Observing System Simulation was done at the Forecast Systems Laboratory. A mesoscale model was used to provide a "nature" field of water vapor. Pseudo observations of path-integrated water vapor from an assumed 40 km resolution observing network, with appropriate error fields, were calculated from the nature field. Three dimensional variational analysis was used to recover the full three dimensional field of water vapor. Results from this preliminary experiment indicate that such a network, with resolution from 40 to 80 km, could be useful for real time monitoring of water vapor fields